This invention relates to the reduction of voltage drop in electrical power distribution systems and to decreasing the loss in starting torque of motors supplied by such systems, and more particularly to reducing the voltage drop in such a system during the starting of large multi-phase induction motors and to decreasing the loss in starting torque of such motors.
There are many geographical areas remote from any electrical power generating station and from the nearest substation where large horsepower motors have to be installed. Such motors are needed, for example, in agricultural operations such as for grain driers and for powering pumps for irrigation, and also for oil wells and pipe lines, etc. These motors range from at least 25 and 50 hp. to 100 and 200 hp. sizes and may have to be located 5, 10, or perhaps 50 or more miles from the nearest substation. When such a highly inductive heavy motor load is initially energized, a sudden and exceedingly high inrush current is drawn (typically five to six times the full load current drawn by the motor while running) at a very low power factor. This high initial current gradually diminishes and the low power factor at starting improves as the motor speed rises to its normal operating or running level. This high inrush current principally comprises reactive current, or magnetizing current to generate the magnetic field required for operation of the motor. It causes a sag or drop in the line voltage and this is reflected back along the entire distribution system thus adversely affecting all the other customers supplied by that system. This has many ramifications including an attendant decrease in the starting torque capacity of the motor being energized. As the starting torque of an induction motor varies as the square of the applied voltage, even a 10-15% drop in the line voltage, which is far from unusual in such situations, will result in nearly a 20-30% loss in starting torque. Such loss of starting torque typically causes an increase in the time required to bring the motor up to running speed. As this time period will increase in an inverse relationship to starting torque, the excessive current load and voltage sag during motor starting will continue longer than the normal time required at proper line voltage levels. In many rural areas the distribution system, while able to carry the running load current of the system's motor loads, will be unable to sustain, without an unacceptable voltage drop, the starting load or magnetizing current of large horsepower motors as they are brought on the line. Also, this prolonged high current drain can exceed what the local supply wiring to the motor can safely handle. Another serious problem is that the starting torque of the motor may fall below that necessary to pick up its load, for example, a piston-type pump, and the motor will be inoperative.
Another disadvantage is the possible expense to the utility customer, particularly in those areas served by utilities which have rates which provide for extra charges based upon a current demand exceeding a certain level for a given time period each month. As many of the demand meters used by these utilities measure the apparent power in KVA and not real or actual power in KW, and the monthly time period that the excessive demand cannot exceed may be on the order of only a few minutes, a very substantial monthly charge can be entailed by cyclic operation of such large induction motors.
A number of approaches have been made toward resolving these problems. One technique is to energize the motor not directly from the line but through an autotransformer that will reduce the applied voltage to the motor during starting. While the current load and the voltage drop on the system are reduced, the motor power factor is worsened and the starting torque is sacrificed (in proportion to the square of the impressed voltage) by this technique. Moreover, the expense of such a transformer, $100 or more per hp., and the space required to accommodate such a transformer constitute decided drawbacks to this arrangement.
A substantial reduction in starting current may also be accomplished by "incremental" or "part-winding" starting (i.e., initially energizing only a portion of each stator winding and then additional portions), but this also dramatically reduces starting torque and, further, requires internal structural modifications of the motor and expensive switching control devices.
Another approach has been to have the motor stator windings connected in a wye configuration during starting and switching the connections to a delta configuration as running speed is achieved. This will reduce somewhat the starting current drawn by the motor and thus the dip in the line voltage, but again, as in the above approaches, as the effective voltage on the stator winding is thereby decreased the starting torque of the motor will also be sharply reduced. Additionally, the cost of the switching components required add significantly to the expense of this approach.
A further technique proposed to counteract this problem has been to connect capacitors across the distribution line ahead of the customer's distribution transformer which will step down the distribution voltage from say 22 KV to the usual 480 volts. Such a technique is described in a paper by W. E. Shula of the Texas Electric Service Company entitled "Starting Large Induction Motors With the Aid of Shunt and Series Capacitors", presented at the PIEA-PESA-PEPA Conference in Houston, Tex. on Apr. 21-25, 1974. This technique will reduce somewhat the distribution line voltage sag but it will not significantly reduce the loss of starting torque of the motor. This is because the high KVA load (due to the low power factor and high quadrature current during motor starting) is applied to the low voltage or secondary of the distribution transformer and is isolated from the power factor correction effected on the primary by the capacitors. Thus, the high circulating current in the secondary results in significant power losses and can cause a core saturation condition in the supply transformer. Moreover, these capacitors must have voltage ratings sufficient to permit them to be installed on such high voltage lines and therefore are electrostatic-type oil-filled capacitors. As they must be connected and disconnected from the distribution line at the beginning and end of the starting period of the motor, high voltage switching equipment and load protective devices must be utilized. These high voltage components are, of course, quite expensive.
Still another possible way of overcoming the provlems of voltage dip in the electrical distribution system when starting large horsepower motors in remote locations would be to use a soft-start motor. Such motors are described in co-assigned U.S. Pat. Nos. 3,670,238 and 4,158,225 in which the resistance of the rotor is increased so that the in-rush current does not greatly exceed the full load running current. However, such motors involve special rotor construction which increases the cost over that of conventional induction motors which have starting currents 5-6 times running current and are classified under the National Electrical Code by the letters F, G and H. A comparable soft-start motor, which would have a starting current not much greater than the running current and would be classified as a code A motor. Also, these soft-start motors have a reduced starting torque. Moreover, only a very few of thid type motor are produced and in limited horsepower sizes. There are thousands of the conventional code F-H large induction motors in daily use and for these there has been no practical and satisfactory arrangement for counteracting the serious effects of large horsepower motor starting on both distribution system regulation and diminished motor starting torque.